To use a booster pack you use your standard jump than press the jump button again until your energy gauge (looks like yellow and grey bars at bottom left corner of screen) runs out. press the jump button again to cancel

A booster pack is a collectible (looks like a backpack with downward blades and a purple aura) that, when collected, allows you to fly for limited times. it stays until you die then disapears, and does not hinder your movement or other such abilities.